import cv2
import numpy as np

if __name__ == '__main__':
    channel1 = np.zeros((300, 300, 3), dtype=np.uint8)
    cv2.imshow('channel1', channel1)
    channel1[:,:,1] = 255
    cv2.imshow('channel1', channel1)
    cv2.waitKey(0)
    cv2.destroyAllWindows()